move library.db and state.pickle to $XDG_DATA_HOME

these aren't config files and thus shouldn't be in `$XDG_CONFIG_HOME`. It's possible other plugins might store non config stuff there too, but I don't seem to have any of those plugins.

Start by locating where library.db and state.pickle paths are configured and how XDG_CONFIG_HOME is currently used. Confirm the intended XDG_DATA_HOME location, then verify that both files are stored there instead of XDG_CONFIG_HOME.

Written by the indexing model from the issue text.